Resin compositions are widely known which have been obtained by reinforcing polyamides with fibrous materials such as glass fiber or carbon fiber or inorganic fillers such as calcium carbonate. However, these reinforcing materials show poor compatibility with polyamides. As a result, there problems occur in that the toughness of the reinforced polyamide is deteriorated, although the mechanical strength and heat resistance thereof are improved, and a molded article formed of such a resin composition reinforced with such fibrous materials undergoes serious warpage. In addition, there is another problem in that the mechanical strength and heat resistance of a resin composition reinforced with such inorganic fillers cannot be improved unless the fillers are blended in a large amount.
In an attempt to overcome these disadvantages of reinforced polyamides, there have been proposed resin compositions comprising polyamides with clay minerals, a typical example of which is montmorillonite. This concept is directed to producing a composite material where polyamide chains are incorporated between layers of a clay mineral and thus finely and uniformly dispersed therein. For example, JP-A-62-74957, JP-A-1-301750, JP-A-2-866628 and JP-A-3-7729 each disclose a resin composition comprising a polyamide with montmorillonite and a process for producing the same (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
When montmorillonite is to be used for such a resin composition, however, it is essentially required to enlarge the interlayer distance of the montmorillonite by,contacting it with a swelling agent (such as an amino acid) prior to blending the montmorillonite with a polyamide or a monomer capable of forming a polyamide, as is described in the unexamined published patent applications cited above. Therefore, there has been a great need in the art to develop an inorganic filler whereby the disadvantages of conventional reinforced polyamide resins can be overcome without performing the above-mentioned treatment.